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The CEfx SPKR module is a diagnostic tool that allows you to listen to the other modules, either on headphones or a speaker. It was designed to amplify synth- or guitar-processor-level signals. Lower level signals can be accommodated by adding resistors, although it requires soldering.

For stereo signals, send the left signals into either of the LIN testpoints and the right signals into the RIN testpoint. Send mono signals into the leftmost LIN testpoint, then run a jumper wire from the rightmost LIN testpoint to the RIN testpoint above it.

Wear headphones responsibly. Slip them off your ears when powering on or off. When making changes to a setup, start with the volume turned down.

Although practically any speaker will work, best results come from using a high-efficiency full-range 4-Ohm driver. The module's max output power is about 2W into a 4-Ohm load.

Depending on its configuration, your module might come with a switch to select headphone or speaker mode, or a jumper. Operation is the same -- slide the switch or move the shunt to the HPHON setting to enable the headphone, or the SPKR setting to enable the speaker.

I designed this for classroom use, so i wanted students to be able to work with headphones most of the time, then quickly switch to a speaker to demonstrate their work. I also wanted to avoid the power-on pops typical of many headphone amp circuits.

My original design just had a mono signal path, but as the CEfx line of modules grew to include stereo effects, I added support for stereo headphones (but still a mono speaker). For mono sources, the user shorts TP91 and TP78.

I wanted this module to work with pretty much any reasonable audio signals, so I started with a ganged 20K volume control, then followed with non-inverting amplifiers. As shipped, they're configured for unity gain (R84 and R79 not installed), which is about right for synth- and guitar- level signals, but installing those resistors can increase the gain to boost weaker signals, like from consumer audio sources.

The buffered signals are available on TP76 and TP74. They also go directly to stereo headphone amplifier U16 (NCP2811) set up for unity gain (R76 = R81).

The headphone amplifier is one of my favorite chips, the NCP2811. Unlike most single-voltage headphone amplifier chips, it avoids DC offsets and power-on pops by creating a negative power supply voltage. That seems a bit pointless in a circuit that already has -12V, but it's inexpensive and has an enable/disable ability which performs the headphone/speaker switching function and also prevents against power-on pops.

The NCP2811 datasheet gives +5V as a maximum supply voltage, so I my first design used the +5V rail, but I experienced some chip failures that I think were caused by a small overshoot of +5V, so I added U19 to make a nice, quiet 3.3V.

The stereo signals go to summing amp U20A, a standard design, and then into speaker driver U18. The SSM2305, is a well-behaved, inexpensive solution that has the required shutdown mode. I expected this to be used in a desktop situation, with the speaker nearby, so I figured that a couple of watts of power would suffice, and it does seem to do the job. It can fill a classroom when driving an efficient 4-Ohm speaker.

Although the SSM2305 works fine with a single-ended input, I had a spare op-amp, so I used U20B to invert the summed mono signal and create a differential input for the SSM2305. It probably doesn't improve anything, but it makes me feel better.

The ~SD pins on both U16 and U18 are an active-low shutdown. TP59, TP67, and TP79 can be populated with a jumper block, or a small slide switch, or wires to a front panel.

It turned out not to be enough to rely on the built-in pop suppression, because the CEfx +/-12V lines took longer to stabilize than the +5V line, so on power up, the thump created by offset shifts in U15 (and previous op amps in the audio chain) fed through the NCP2811 after it had unreset itself. The solution was to add zener diode D9 and R102 to create an enable signal that depended on the +12V line being mostly stable. This seemed to be enough to get rid of the thumps.

Speaker connector. Responding to universal criticism about the tiny screw terminals of the speaker connector, I replaced it with a spring-loaded connector. This connector can be retrofitted onto the Rev03 PCB, but you will need to cut off the rear pins and put a bit of VHB tape under the connector to keep the stubs of the rear pins to from shorting out to the PCB.

Low-voltage operation. This module deliberately disables the outputs when the +12V rail goes below about 10V. I did this to fix a power-on thump problem. However, several users were interested in using this module with reduced voltages, esp running off of 9V batteries. So I changed the shutdown levels to about 7V, which still seems to keep the thump problem under control, but allows for operation with a 9V battery regulated to 8V.

Aux phone jack. These modules never seem to have enough phone jacks. Some users want to send signals to an off-board effect box, then route the returns through the SPKR module. Other users want to take the line-level aux outputs (LOUT, ROUT, MONO OUT) and send them off-board. For both of these cases, I added an uncommitted stereo phone jack, which can be wired either to inputs or outputs.